[flang] runtime: fix WRITE after BACKSPACE on variable-length file

BACKSPACE leaves "recordLength" set, which is fine for a later READ,
but it causes a later WRITE to fail due to a misinterpretation of the
knowledge of the record length as indication of a fixed-length record
file (RECL=).  Fix.

Differential Revision: https://reviews.llvm.org/D108594